MPG = Air Resistance vs Speed vs Weight

My head hurts already, I can see the theories that this thread is going to generate. This formula might work well on paper, but there's too many variables in the real world that aren't factored in.

According to your formula I am getting twice the fuel economy than I should. TT 11.5K#s, TV Ram 6.7L Cummins, TT 11.5' high towing at 65MPH over 18K miles, Fuel mileage, 11.75 MPG.

hddecker wrote:
TT 11.5K#s, TV Ram 6.7L Cummins, TT 11.5' high towing at 65MPH over 18K miles, Fuel mileage, 11.75 MPG.

The formula is conservitve, you should have a aerodyanic trailer. A semi is a flat wall (.6), and a 5th wheel has some curves (.5 or even .4).. so it's not perfect unless you put it in a wind-tunnel to know your true drag.

11.5' did you remove 1 foot for road-clearance.

10.5' * 7 wide= ~75ft
Diesel=.3 eff or perhaps .4 with a turbo charger
Lets say your tt has some good curves, 0.4 drag
11.5k lbs weight, with .4 drag

@65 MPH = 10.95 MPG (.3 engine) / 14.6 MPG (.4 engine)

You'll have to tweak the numbers until it fits your rigs MPG, then change your weight and see how little it changes.

SplinterFL wrote:

hddecker wrote:
TT 11.5K#s, TV Ram 6.7L Cummins, TT 11.5' high towing at 65MPH over 18K miles, Fuel mileage, 11.75 MPG.

The formula is conservitve, you should have a aerodyanic trailer. A semi is a flat wall (.6), and a 5th wheel has some curves (.5 or even .4).. so it's not perfect unless you put it in a wind-tunnel to know your true drag.

11.5' did you remove 1 foot for road-clearance.

10.5' * 7 wide= ~75ft
Diesel=.3 eff or perhaps .4 with a turbo charger
Lets say your tt has some good curves, 0.4 drag
11.5k lbs weight, with .4 drag

@65 MPH = 10.95 MPG (.3 engine) / 14.6 MPG (.4 engine)

You'll have to tweak the numbers until it fits your rigs MPG, then change your weight and see how little it changes.

Nope, it's like a great big brick, absolutely no aero to it at all. The front wall is almost straight up, might be 2-3 degrees off vertical.

Doesn't the fact that the TT is up in the air actually increase drag. Seems to me that the NASCAR engineers work as had as they can to stop the air from getting under the car and create drag. A lot more semi trailers a being equipped with the side skirts, must have some thing to do with stopping aero drag.

This calculator is better than nothing but judging from the inputs other factors are not accounted for.

weight is in most cases a secondary effect in towing mpg. It comes into play when accelerating or when going up/down a grade. At a constant speed on level ground weight is a very secondary effect. Wind resistance is by far the dominant,effect.

If your going over very hilly terrain or stop and go traffic, or constant changing speeds, then the weight can have a effect. It's effect is 1/2MV^2 where M=mass, V= change in velocity. Or if your going up/down hills its related to M*d Where M=mass and d= the CHANGE in elevation. If V is constant and d=0 mass doesn't have much effect. It increases rolling resistance and a few other minor effects on economy.

I have two trailers, one a 7'wide cargo trailer 5,000lbs loaded. the other a 9,000lb 8' wide TT that is also about 2' taller. Difference in mileage over the same course is rather small. Towing the "big" one I get 10-12 depending on mountains or not. Towing the smaller one is 11-13, occasionally 14 on very flat ground on the interstate at constant speed. Roughly a 10% hit going from the small frontal area, low weight to the large frontal area and a big weight increase.

also, to do a calculation of speed effects you REALLY need to know the Cd and actual frontal area. not a trival task, even a square wall trailer front has a Cd well below one, the longer the lower it is for the same frontal area, and some of the frontal area is blocked by the TV.

The calculations shown by the OP show a mpg less than 1/2 of what I get pulling a 10,000 trailer with a 9,500 lb truck at 60ish.

ktmrfs wrote:
weight is in most cases a secondary effect in towing mpg. It comes into play when accelerating or when going up/down a grade. At a constant speed on level ground weight is a very secondary effect. Wind resistance is by far the dominant, effect.
.

Absolutely correct.

FWIW my 8500lb TT gets me the same mpg as my 2500lb v-nose snowmobile trailer. OK, maybe .5-1 mpg. So as far as I'm concerned, weight means nothing and frontal area means everything. And v-noses gain you absolutely nothing! LOL

I agree with mooky. Frontal area is everything. That's why you will get better mileage from a Hi-lo than a conventional trailer.

I always like it when science confirms what I have established empirically.:W

Speed and big frontal area are bad but you can also add wind, hills, fast accelerations, gas or diesel power and octane content of the gas to your mileage variation.

By the time I drove the 50 plus miles from the dealer to home, I knew I would get 9 to 10 mpg. It has not varied much from there.

One weekend I drove to a camp ground in very strong head winds and barely managed 8.5 mpg. :E going home with tail wind brought 10 plus mpg.

Those who want good gas mileage generally stay with a Pop-up, egg or other more compact camper.

In the end, convenience costs. It is the nature of the TT beast. :)

When the gas guage reads a quarter tank, it is time to find a gas station. It is what it is.

The formula doesn't seem to take wind speed into consideration. Recently, going west on I10 I used up much more gas than returning eastward.

It's not an exact science - your results WILL vary from trip to trip.